A Comprehensive Understanding of Printing Process to Improve Prepress Production Level (2)
three. The role of offset printing and its transfer in the printing process
The dot is an important concept of printing technology. The parameters of the dot mainly include the number of dots (dot density), dot angle, and net shape.
The number of meshes refers to the degree of detail of the dots, generally expressed in lines/inches (lpi). In fact, this name is based on the concept of photoengraving. The standard unit should be dots/cm. According to the different types of paper, the number of meshes used in the outlets is also different. Newspapers and other newsprint products generally use 85-100 lpi, and the main body of the books and other offset paper products use about 130 dpi, while magazine covers and high-grade paintings use 150-200 lpi.
Color prints are overprinted with four colors of ink. In order to make the printed products more fine and do not produce moire, after practice, the screen dots of the four color versions have to be interlaced. The dot arrangement is the angle of the net. Generally, there are different angles such as 15°, 45°, 75°, and 90° (0°). The selection of these angles is practiced in accordance with the visual habits of the human eye.
Dots have different shapes, such as circles, squares, diamonds, chains, etc. These meshes are adapted to different manuscript features, and the general products are mostly round dots.
The network points mentioned here are amplitude modulations, which represent the hierarchical tone based on the changes in the dot size. In recent years, there has been a method of frequency modulation screening. The network points are extremely small, the size of the network point is constant, and the frequency of its distribution is changed to obtain the level change. At present, this technology is still in the trial phase.
Printing technology can present rich and colorful images to readers. It can display various levels and adjustments on photographs in printed matter, enabling colorful colors to be reproduced. The method used is to use the size of dots with the same darkness. Changes through the visual characteristics of the human eye to express the different levels of tone, the use of yellow, product, green, black color pigment subtractive method to form ever-changing colors.
Nature's colors, levels, photo colors, and tone levels are all represented by changes in shades and shades of color. The hue and shade of ink used in the printing industry are fixed. Therefore, the concept of network points is introduced here. That is to say, the entire screen is divided into small uniform patches that are almost indistinguishable from the human eye. The more concentrated the color of the block, the greater the percentage of ink in the block. According to the theory of subtractive colorants, various colors can be produced by mixing four colors of yellow, magenta, cyan, and black. For example, 100% green plus 100% yellow can produce green. In the pre-press work, we have scanned a color image and digitized it into a CMYK (Huang Pin Qing Black) format. The image consists of four color versions of Huang, Pin, Qing and Hei. When the film is output, the corresponding film is output to print four PS plates corresponding to the four color inks.
Look at the process of color conversion from nature to print. Natural colors are their own true colors. Colors are represented by photographic paper pigments using photographic techniques, or digital images are taken directly from digital cameras; CCD photosensors are scanned using scanners. Or photomultiplier picks up the photo image information, converts the photo data into an RGB image represented by 256 shades of gray, then converts the RGB format to the CMYK format required by the printing industry and adjusts and corrects the image. At this time, we refer to the screen. The color is displayed by RGB using the color light additive method. By calculating RIP, 256 gray scales are used to generate 0-100% dots on the film, and the output is in the black and white binary value corresponding to the four colors C, M, Y, and K. On the film, after printing and printing, the ink of the corresponding color is given to the network and transferred to the paper, and various colors are formed by subtractive coloring of the color material. In this process, from the nature to photos, data files, films, prints, the tone range should be reduced once, such as the picture is 2.5D, the positive film is about 4.0D, due to the scanner's general flatbed scanner Can only pick up 2.2D, drum scanner up to about 3.5D; the density of the image on the film in the range of about 3.0D; in the print, coated paper density in the range of 2.0D, newsprint only 1.2-1.3D . The process of print copying is the process of compressing the image tonality. From the perspective of color gamut, RGB is smaller than the color gamut of the natural world, and CMYK is smaller than the color gamut of RGB. Therefore, many bright colors cannot be reproduced.
At present, prepress technology is mostly used in the back-end hanging net, which is generated when the RIP outputs a film. As the constituent elements of color and gradation, outlets require that the consistency of products should be maintained from generation to printing. This is the focus of color management. The expansion of outlets at the time of printing is unavoidable, but due to the predictability of the expanded value, its influence can be appropriately compensated before adjusting the color separation before printing. In the actual work, the accuracy of the dots generated on the control film and the consistency of copying from the film to the PS version are the key to the dot control.
four. Printability of paper, ink, and printing presses
Paper, ink, and printing presses are the main raw materials and tools for printing technology and are one of the most important factors affecting printing quality. Therefore, prepress operations must have a comprehensive understanding of this.
Paper has a very important role in the quality of printed products, its parameters are mainly smoothness, whiteness, moisture content, ink absorption, tightness, degree of sizing, weight, thickness and so on. The factors to be considered in prepress operations are the following:
1. Determine the range of network cables that can be attached to the network.
2. Determines the range of tones to be controlled when color adjustments are made at the picture level.
3. Effect on separation parameters. The total amount of ink, the amount of black print, and dot gain are determined by the paper.
4. The effect of color adjustment on the picture level. Consider the whiteness of the paper, the absorption and reflection of the ink.
5. The moisture content of the paper affects the geometric stability of the paper during the printing process. After the printing paper absorbs part of the fountain solution, its geometric dimensions will change, especially in the fan-shaped expansion phenomenon in the printing press paper.
Ink is the main basis for printing color gradation and reproduction of graphic and graphic texts. By using inks, we have obtained prints with rich colors and bright colors. The parameters of the ink are mainly hue, saturation, purity, fluidity, viscosity, absorption, transparency, fineness and so on. The ink factors to be considered in the prepress operation are:
1. The actual hue of the ink has a certain gap from the theoretical value, and the error range is different due to the different hue inks.
2. The hue values ​​of inks of different varieties and brands are different.
3. Different types and brands of inks have different drying times and dot gain rates due to their different fluidity, viscosity, and absorbability.
4. The background color and text color of platemaking must be taken into account in the hue of the ink, and it should be selected from the "print chromatography" when designing.
5. A large area of ​​background color is best not printed on the ground, but overprinted with 95% of the dots. For example, 95% black and 95% green overprint can be used as a black background.
6. In the use of overprinting overlay text on the background, to consider overprinting after the color effect, such as overprinting the magenta text on the light yellow, etc.
The printing presses are mainly divided into the following categories: sheet-fed monochrome printing machines, sheet-fed two-color printing machines, sheet-fed four-color multi-color printing machines, newspaper rotary printing machines, and commercial semi-commercial rotary printing machines. Due to different printing methods and printing processes, it will also affect the prepress process parameters. This mainly has the following aspects:
1. The registration of multicolor products for printing presses is relative and is constantly changing during the printing process. There are certain differences in the overlay error range of various printing presses. The high-grade import four-color machine is better than the domestic four-color machine, the four-color machine is better than the monochrome machine, and the sheet-fed printing machine is better than the web reel printing press, etc. Wait. It is necessary to understand the printing equipment in prepress production.
2. The overprint error shown by the printed product is not just the error brought about in the printing process. The errors in the process and positioning system such as film, imposition, printing, etc. will be reflected in the printed product.
3. The printable web of a printing press is fixed, and the print size of its products, including various lines, cannot exceed its range of adaptation.
4. The color settings of the text on the layout are mainly simple and neat, and the color registration should not be too complicated, so as to avoid illegible writing due to overprint errors. Especially for colored newspaper prints, the larger numbered headline characters can be overprinted in two colors, and the smaller font size is preferably in monochrome. For overlying or anti-white characters on the background, the font decoration should not be too complicated. The anti-white trumpet characters on the background, if the background color is multi-colored, should take into account the problems arising from overprinting errors. In newspaper printing, monochrome shading should be used to reverse white.
Fives. Binding process requirements
Most printed products are to be bound or folded into booklets, and single-page prints are also cut or die-cut. The binding folding method and die-cutting process require the close cooperation of the pre-press process. The pre-press work should be organically coordinated with the binding process after the page, page number, printing center, bleeding, and labeling. In the past, these tasks were mainly completed by plate-making processes. With the increase in the size of laser image-setters and the application of direct plate-making technology, a considerable part of the cooperation has been completed by the prepress imprinting process. In the future, such work will be more.
As for the binding methods, there are mainly saddle stitching, flat stitching, lock stitching, glue binding, and hardcover. Before book binding, the first step is to fold the pages, which is to fold the large-format sheets into the signatures according to the specifications and page number. The folding is divided into two methods: manual folding and mechanical folding. The folding modes include vertical cross folding, parallel folding and mixed folding. In order to facilitate the quality control in folding, the large-format printed sheets are printed with signatures and polylines.
The requirements of the binding process for prepress operations mainly include the following aspects:
1. In a single page to accurately determine the finished product version of the center, finished product size and husband bleeding, there must be a full set of crosshairs, tangent lines.
2. According to the requirements of the format and folding process of the laser imagesetter, the entire page can be assembled into a full-page output, which can effectively improve the output efficiency and print binding quality. According to the folding and binding to determine the page order and the layout of the layout of the layout of the size of the center, to give a complete crosshairs, tangents, fold lines in the layout, the cover should be given exactly the thickness of the spine.
3. When using a large-format photo-setter and a direct plate-making system, the printing and binding process rules and markings are completely completed by a prepress production process. Under the premise of clarifying the folding method and binding method, we must accurately determine the page number order and provide the content required for binding, craftsmanship, labeling, edition, nibbling, dragging and other printing and binding processes.
4. Consider the cutting position when designing the drawing, and consider the effect of the folding binding method.
It is difficult for prepress workers to completely understand and accurately present the various technological contents that meet the requirements of printing and binding. We should realize that with the continuous advancement of prepress technology, the traditional platemaking process It is shifting to the prepress process step by step. There will be more work in this area in the future. This will require the prepress workers in the future to learn more about the techniques and requirements of printing and binding, and continue to learn to adapt to the prepress technology. Development requirements.
six. How to learn more about printing process technology at work
The prepress staff's understanding of printing opportunities is limited, and it is not practical for everyone to go to the process of printing and binding. There is actually no need for this. So, what kind of methods and ways should prepress staff learn more about printing technology?
1. In the work practice, we have a lot of people, and we continuously summarize the requirements for cooperation before printing and post-processing such as printing and binding, so as to deepen our understanding of the printing process technology. First of all, prepress staff should use themselves as printing industry
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